Fix/filmstrip multiselect anchor

[savukadoo]

Description

Fixes an issue where multi-range selection in the filmstrip incorrectly expanded into a single continuous range.

Type of Change

• Bug fix

Changes Made

Previously:

• Shift selections always anchored from the initial selection
• Resulted in unintended selection of all items between first and last

Now:

• Introduces a dedicated selection anchor
• Supports proper additive multi-range selection (e.g. 1–2, 5–7, 9–11)

Screenshots/Videos

Testing

• I have tested these changes locally and confirmed that they work as expected without issues

Test Configuration:

• OS: macOS Sonoma
• Hardware: Apple M2

Checklist

• My code follows the project's code style
• I haven't added unnecessary AI-generated code comments
• My changes generate no new warnings or errors

AI Disclaimer:

Please state the involvement of AI in this PR:

• This PR is AI-generated but guided by a human

[CyberTimon]

Thanks for the PR! Looks great. Can you remove the unrelated commit from the other PR? I will merge it then 💯 . Thanks for your contribution :)

[savukadoo]

apologies, I have removed that unrelated PR and it should be cleaner now

[CyberTimon]

Thanks! Will merge now. Appreciate all your contributions 💯